fenix 5x & hrm run HR

Hello,

I have a Fenix 5x and a HRM-run HR belt (the one with the white runner printed on it).

I also have a Fenix 3.

When I search for the belt with the Fenix 3 it detects it after a few seconds, no problem.

But When I am doing the same procedure with the Fenix 5x, nothing happens.

I updated the watch to the last version via garmin express. After that I go into settings and

search for a new FC sensor. But it tells me to search again as nothing was found.

Again, works flowless with the Fenix 3.

Anyone had this kind of issue and find a way to resolve it ?

Thanks in advance.